Many people arrive late, while the theater is already dark. It is easier to grab an empty seat than fumble around trying to identify the right number. Some theaters used to have ushers with flashlights, but they wouldn’t want to pay them now.
Well, the solution is simply, don't be late and don't disturb all the others.
I also am quite late in many situations, but in a theather you just go, if you can manage to be seated, before it gets dark. Everything else is just rude.
